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49 Cards in this Set

  • Front
  • Back
contrast perception is the chief job of what?
the retina
why can one read black text on white paper in the bright of day, in twilight or inside a normal room?
contrast perception by the retina is intensity independent over a wide range.
perception of features occurs where?
in the cortex
what does retinotopical organization mean?
where a cell is located in the retina will dictate the visual location of the stimulus when that cell is stimulated and the stimulus is visualized
what are the large ganglion cells of the retina called and how many layers of the LGB do they terminate? Small ganglion cells?
large ones are magnocellular and terminate in 2 layers of the LGB. Small cells are parvocellular and terminate in 4 layers of the LGB.
the outer segments of rods and cones contain what and how are they organized?
they contain an opsin (cones) or rhodopsin (rods) which anchor retinal on the outside of the lamellae (disc membranes inside the cell body).
why do cones provide color information?
they contain 1 of 3 color opsins and they are precisely arranged as opponent pairs in the receptive field of P type ganglion cells.
what is the order of opsins and rhodopsin in terms of increasing wavelength (from shortest to longest) and what color is the main one detected?
short opsin (blue), rhodopsin (black and white), medium opsin (green), and long opsin (red)
what cone cells will respond to red, green, and yellow?
the red and green cones will respond to all these colors bc their wavelengths are similar.
how are the rod and cone opponent pairs organized?
in concentric circles containg red cones and green cones (red v. green), blue cones and red or green cones (blue v. yellow) and any cone and rod (white v. black)
color perception defects may be located where?
in the cone pigment (retinal location usually inherited) or cortical bc the cortex is required for discrimination.
what is meant by no convergence of signal and where does this occur?
the cones of the fovea connect with single bipolar cells which connect with single ganglion cells.
describe the photosensitive cells of the fovea?
only long, slender, spread apart (thus they are hit directly by light) cones are found in this area
what ganglion cells provide the best visual accuity?
the P cells of the center of the retina (macula area) called midget cells bc they are the smallest ganglion cells
what characteristic of cone cells give them good temporal resolution?
they fire at about 55 Hz (55 times per second)
cones work best under what conditions?
under lit conditions; they are photopic
what qualities of the cells in the fovea make them produce a sharp image?
they have no signal convergence, small receptive fields, and thus a good spatial resolution
what happens to the discs in rods over time? cones?
they are continually shed in rods and reabsorbed by the pigment epithelium in cones
what photoreceptor cells are more numerous?
rods (100 million vs. 6 million)
do rods function better in dim light or bright light?
in dim light, they bleach out in bright light, thus they are considered scotopic vision cells for night vision
What properties of rod cells make them better white light amplifiers than cones?
they have a lower temporal summation (12 hz) thus they can sum more information and they have longer outer segments with more discs. This is why they function well in dim conditions, they can absorb all the light.
why is the rod based image grainy but more sensitive to light than cones?
multiple rods synapse onto one bipolar cell and multiple bipolar cells synspse onto the M type ganglion cells which are larger and have larger receptive fields
describe the electrophysiological events during darkness in the eye?
the rods and cones are depolarized. cGMP is constantly produced and keeps the cGMP gated ion channels open, thus 90% Na and 10% Ca flow into the cell and depolarize it. Note K+ flows out of the discs into the cytoplasm to complete the circuit.
when light hits the rod or cone, what happens immediately?
11 cis retinal in the discs is isomerized to all trans retinal very quickly.
what does all trans retinal signal to occur?
it stimulates a transducin (G protein) to activate a phosphodiesterase that hydrolizes cGMP, thus the ion channels close and the cell is hyperpolarized via regular potassium mechanisms.
describe vertical processing for a cone. a rod?
cone to bipolar to ganglion cell. rod to bipolar to rod amacrine cell to ganglion cell.
what is the purpose of the gap junctions between rods and cones at their bases?
they permit greater sensitivity of rods in dim light, allowing the rods to also use the cones vertical processing pathways thus allowing more sensitivy in dimlight, but once it is too dark, you switch to a rod based system only (the gap junctions close). Thus you can only see shades of gray and not color.
Horizontal cells provide what? When do they close?
They increase the receptive field surround and close at twilight, thus the image becomes grainy.
Amacrine cells are found where and what is their main function?
Modulate bipolar cells to ganglion cells and process laterally. Amacrine cells are especially sensitive to motion (especially A2 amacrines)
What do off center bipolar cells do when light hits their rod or cone?
Light hyperpolarizes the rod or cone to stop sending AP’s and thus stop releasing glutamate. Thus the off centered bipolar are disfacilitated, they receive less NT from the rod or cone and are stimulated less in light. Thus they detect less light.
What do on center bipolar cells do when light hits their rod or cone?
Glu release to the metabotropic mGluR6 receptor on on center cells causes hyperpolarization of these bipolar cells; thus light causes less Glu to be released from the rod or cone, thus the on center bipolar is disinhibited (it turns on), and is depolarized to fire a message.
Why do on and off center bipolar cells respond differently to Glu?
They have different Glu receptors. On centers have metabotropic while off centers have ionic receptors.
What is the basis for the contrasting effects of retinal cells?
They are organized into on center, off surround or off centered, on surround mosaics which are very sensitive to differences in intensity between the center and the surround.
When only the surround is illuminated in an antagonistic center v. surround scenario, what type of response do the ganglion cells exhibit and how do they do this?
The horizontal cells cause the bipolar cell in the center to express opposite ganglion responses. The horizontals do this via GABA.
What are the two theories used to describe color vision?
The trichromatic theory and the opponent processes theory.
Which ganglion cells carry color contrast info? Brightness contrast info?
P cells for color and M cells for brightness
What cells in the cortex can distinguish color from brightness and why can they do this?
Double opponent cells can dp this bc of synaptic convergence
What retinal ganglion cells have a faster conduction velocity and have some fibers that synapse in the superior colliculus as well as the lateral geniculate nucleus?
The M cells
What are the 6 pathways that the P,M and O type ganglion cells give rise to? Where do they go? What do they do?
the geniculostriate pathways are pathways 1 through 3 and follow the normal path to the LGN to the visual cortex. These pathways are how we make sense of output from the retina. The extra striate pathway is #4 which uses M and O ganglion cells. It goes through the superior colliculus and the pulvinar to synapse on pariteal and temporal lobes. It is used for crude imaging to orient scene info. The midbrain visuomotor pathway is # 5. It involves M and O ganglion cells. It mediates eye and pupillary reflexes and its path is confined to the midbrain. Pathyway #6 is to the suprachiasmatic nucleus involving O type ganglion cells and it regulates our circadian rhythm.
how is the visual cortex organized cellularly?
they are organized into columns of cells that detect different stiumlus features that are arrayed precisely and are only interupted by blobs.
What are the functions of the blob and interblob areas in the cortex.
The P blobs sense color. The P interblobs sense color, orientation, and depth. The M interblobs receive orientation, depth, and motion
what are the two types of columns in the visual cortex? locations? functions?
the orientation columns found in the primary and secondary cortex provide complex shape info (they have different orientation angles they respond too). The occular dominance columns of the primary cortex provide binoccularity (columns are arranged so that a cluster receives one eye input and another will receive other eye input) and depth of field (stereopsis).
simple cortical cells do what?
they have large receptor fields in the LGN and process stimuli via occular dominance and orientation selectivity to form "stripes" of light; note it is sensitive to both the position and the angle (orientation) of the light. Thus different simple cells have different orientation angles that they respond to throught 360 degrees.
complex cortical cells do what?
large receptive fields again in the LGN and use light as stripes like simple cells, but are all excitatory unless the stripe is too long and hits nearby inhibitory areas.
dorsal pathways from the primary visual cortex follow what path to end up where? What is the function of this?
Only M cell projections follow this pathway from V1 to MT (medial temporal) or V2 to V3 to eventually V5 and thus the posterior parietal cortex which is used for spatial processing.
Ventral pathways from the primary visual cortex follow what path to end up where? What is the function of this?
P and M cell projections follow this pathway (blobs and interblobs). V1 to V2 to V4 and thus the inferior temporal cortex for resognition of what something is.
If area MT is destroyed, what happens?
movement agnosia
If the fusiform face area in the inferior temporal cortex is destroyed, what happens?
prosopagnosia: ppl cannot recognize faces.
what is achromatopsia?
loss of discrimination of colors, even though the retina is working OK. It is in the cortex where the problem is located